Various gas mixtures are available from natural sources as well as in products and by-products of known chemical processes. A number of different techniques have been employed or proposed for recovery of one or more individual components from such gas mixtures, including among these: absorption in selective solvents, cryogenic fractionation, selective adsorption by solid adsorbents, and certain combinations of such unit operations.
Natural gas, for example, typically comprises in the order of from about 50 up to about 90 volume percent methane, with about 4% or 5% and up to 40%-50% C.sub.2 -C.sub.6 hydrocarbons, up to about 5% or 6% hydrogen, and the remainder made up chiefly of carbon dioxide (less than about 1%) and nitrogen (less than 1/2%). Among known industrial uses for natural gas, other than its use as fuel, is as a source for production therefrom of hydrogen and other valuable products.
Among conventional processing of natural gas or other methane-rich gas mixtures is that of reforming, particularly that carried out by reaction with steam, according to the equations: EQU CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2 EQU CH.sub.4 +2H.sub.2 O.fwdarw.CO.sub.2 +4H.sub.2
The separation of hydrogen from the oxides of carbon in the reformate by present technology is relatively costly from the stanpoint of initial capital investment as well as operating costs incurred in the energy-intensive methods employed. Conventional processes generally combine treatment of the starting mixed gas charge with a selective solvent absorbent, such as monoethanolamine (MEA), to remove the bulk of the CO.sub.2 followed by cooling to cryogenic temperatures for separation of the carbon monoxide from hydrogen. Regeneration of the absorbent requires considerable energy input, which is largely obtained from process waste heat that would otherwise be available for production of steam.
Since small amounts of both water and CO.sub.2 still remain in the process gas effluent from the absorber, it is necessary to provide sorbent driers in the system to prevent these species from freezing out in the gas chilling equipment (cold box). These driers further intensify the already significant capital costs.
As illustrated in FIG. 1, the conventional "state-of-the-art" system for separation of individual components from a gas mixture comprising H.sub.2, CO, and CO.sub.2, such as a gas mixture derived by steam reforming of a methane-rich gas composition, entails the use of a solvent absorbent (generally monoethanolamine) for removal of CO.sub.2, a so-called "cold box" for bulk separation of the carbon monoxide from its mixture with hydrogen at cryogenic temperatures, and a solid adsorbent unit (PSA) for further purification of the crude hydrogen evolved. The crude hydrogen product obtained from the cold box is about 98% pure and may be brought to about 99.9% purity in the PSA unit. The described operation typically provides about 85% hydrogen recovery, the reject gas being utilized as fuel in the reformer furnace.